If you are talking about the language in the iTunes Store, it is set by the country you set your account in. For example, if you set up your account or changed your account to Japan, there would not be any English option. Just like there is no Japanese option for an USA iTunes Store account.
If you're referring to the application itself, like the menu options, that would be in the settings on your device. The best thing to do is to google the settings page so you can see the order of the buttons and follow them back to the language setting.